1. Field of Invention
The present invention relates to fiber reinforced thermoplastic composites, and more particularly relates to thermoplastic composites having reinforcing fibers which have a polymodal distribution.
2. Description of the Related Art
Aqueous methods of making fiber reinforced composite materials from an aqueous slurry of, solid polymer and reinforcing material are known. See Published European Patent Applications 0,148,760 and 0,148,761, Wessling et al., U.S. Pat. No. 4,426,470 issued Jan. 17, 1984 and Gatward et al., U.S. Pat. No. 3,716,449 issued Feb. 13, 1973, all of which are incorporated herein by reference. In general these reinforced polymer composites have a uniform mixture of fiber, polymer and optionally binder and are prepared by performing dilute aqueous slurries of a solid heat-fusible organic polymer, a reinforcing material and optionally a latex binder.
Reinforcing materials disclosed have included organic and inorganic products such as graphite, metal fibers, aromatic polyamides, cellulose and polyolefin fibers, and typically have included glass fibers such as chopped glass strands having a length of 1/8 to 1 inch (about 3.2 to 25.4 mm) or milled glass fibers which generally have a length of about 1/32 to 1/8 inch (about 0.79 to 3.2 mm). These reinforcing fibers have typically involved unimodal fiber distributions resulting in the final composite article after having been stamped under high pressures and at elevated temperatures exhibiting a relatively low impact strength if short fibers are employed exclusively, and exhibiting an undesirably high resistance to flow during stamping at elevated temperatures if relatively long fibers are used exclusively.
Accordingly, there is a need for thermoplastic composites and processes for the making thereof, which result in thermoplastic sheet-like structures which exhibit desired levels of flow during stamping at elevated temperatures and pressures, and which exhibit desired levels of impact strength in the final composite article.